OCEANESS 410 8 Ocean Crustal Structure and Seismic

OCEAN/ESS 410 8. Ocean Crustal Structure and Seismic Reflection William Wilcock (via Michelle W. )

Lecture/Lab Learning Goals • Know the basic structure of the oceanic crust as determined from ophiolite studies • Understand how oceanic crustal structure is linked to mantle melting and volcanic processes • Physics: Understand what seismic impedance is and how it controls the amplitude of seismic reflections • Data collection: airguns & LOTS of hydrophones • Data processing: Be able to explain how reflection data is stacked and converted into a seismic record section • Interpretation: What can seismic records tell you? (LAB)

Structure of oceanic crust: what’s down there?

Geophysics VS Geology • What is the difference? ? – Geology: a science based on direct observation of terrestrial formations – Geophysics: finding things underground without digging. Or diving.

Drilling to the mantle

The Geology part: Ophiolites • Ophio = Snake (i. e. green) • Lithos = Stone

Ridg uca de F Juan North American Plate e What if the Juan de Fuca Ridge collided with the North American Plate? I’m so hungry! I think I’ll eat the whole Juan de Fuca plate. Nom nom Oh no! Please don’t eat me!

What if the Juan de Fuca Ridge collided with the North American Plate? North American Plate I’m so hungry! I think I’ll eat that Mid Ocean Ridge. Juan de F uca Ridg e You would get an ophiolite! - Often happens when ocean basins close Nom nom - You get “obduction”, or upthrust ofnom oceanic crust and mantle onto continental plates Oh no! Please don’t eat me!

Ophiolites around the world

Melting beneath mid-ocean ridges The melt rises out of the mantle because it is buoyant (less dense than the mantle)

Ophiolite Sequence

Mantle

Crust-Mantle Interface (“Moho”)

Gabbro layer

Depth, km 0 3 6

Gabbros (layered) - they can also be un-layered (massive)

Sheeted dike layer - “feeder channels” for basaltic lava rising from below - The cracks extend over large distances beneath ridges

Sheeted dike layer

Sheeted dike layer

Pillow basalt layer

Pillow basalt layer - Lavas extruded onto ocean floor at a spreading center

Sediment layer Pelagic sediments deposited above oceanic crust basalts

Seismic Reflection

Reflections from Interfaces When a downgoing P-wave meets an interface, a portion of the wave is reflected.

Characteristic acoustic impedance - An inherent property of the medium - Depends on sound speed (or seismic velocity) and density - Pressure generated by vibration of molecules of a particular medium at a given frequency

Amplitudes of Reflections for vertical rays The amplitude of the reflected and transmitted phase depends on the seismic velocity, V and the density, ρ in each layer. A 0 V 1, 1 V 2, 2 Larger contrasts in impedance result in large amplitude reflections

Marine Reflection Seismology - Airgun Sources Reflection data is relatively easy to acquire in the oceans. Seismic sounds (shots) can be generated with arrays of compressed air guns (airguns) towed behind the ship

Marine Reflection Seismology - Hydrophone Streamers The airgun shots are recorded by arrays of hydrophones towed behind the ship in a streamer. The seismic streamers contain 1000’s of hydrophones and can be >10 km long. A modern 3 -D seismic ship will tow several (the records is 20) streamers.

Marine Reflection Seismology - Geometry The streamer records waves reflected from interfaces

Normal moveout drawing on board…

Marine Reflection Seismology - Data The seismic data recorded for a particular shot will display a geometric effect termed “normal moveout” (NMO) which reflects the increased distance the wave travels as source-receiver offset increases Time, s 0 Offset X

Marine Reflection Seismology - Sorting Records The records are sorted so that they all have the same mid-point (Common Mid-Point CMP)

Marine Reflection Seismology - Airgun Sources The seismic records can be corrected for geometric affects and stacked (summed) to produce a single record for the reflections below each point Before Geometric Correction After Geometric Correction Stacked (summed)

Marine Reflection Seismology - Filled Wiggle Plots Time, s Stacked records are plotted on the same plot with the horizontal axis showing position along the profile. Rather than showing lines for each record the plots often show filled regions for positive (or negative) displacements Position

A reflection profile across the East Pacific Rise Reflections come from the seafloor, the base of layer 2 A (pillow basalts), the axial magma chamber (AMC) and the Moho (M)

Reflection profile - Sumatra

Endeavour segment of Juan de Fuca Ridge

TWTT (s) Endeavour segment of Juan de Fuca Ridge 2 A/2 B AMC

Intersecting Record Sections from the East Pacific Rise